1. Field of the Invention
The present invention relates to a joint imaging apparatus.
2. Description of Related Art
Images taken by magnetic resonance imaging (MRI) have mainly been used for diagnosis of rheumatoid arthritis because conventional radiographic techniques using an absorption contrast method cannot create images suitable for diagnosis of cartilages in joints of limbs and soft tissues around joints.
Unfortunately, MRI forces a heavy burden on a patient because of holding a posture of the patient under restraint mechanically for a predetermined time period and costs a great deal of money; hence, technology is needed that diagnoses rheumatism using X-ray images taken with common X-ray tubes.
For example, radiography by phase contrast imaging has been proposed as a substitute for MRI (e.g., see Japanese Patent Laid-Open Nos. 2008-023312, 2007-268033, and 2008-18060).
The phase contrast imaging can produce X-ray images with high visibility based on edge enhancement utilizing the refraction of X-rays in the phase-contrast magnification radiography.
Talbot or Talbot-Lau interferometers utilizing the Talbot effect have also been studied as phase contrast imaging (e.g., see Japanese Patent Laid-Open Nos. 58-16216 and 2007-203063, and WO2004/058070). The Talbot effect refers to a phenomenon of coherent light that passes through a first grating having regularly aligned slits, in which the image of the grating is formed at regular intervals in the advancing direction of the light. The grating image is called “self-image,” and the Talbot interferometer measures interference fringes (moire) occurring by slightly shifting a second grating disposed at a position where a self-image is formed. Since a subject in front of the second grating causes the distortion of the moire pattern, the subject is disposed in front of the first grating and irradiated with coherent X-rays during radiography with the Talbot interferometer, and then an image of the subject can be reconstructed by arithmetic processing of the resulting moire image.
Furthermore, a method using the Fourier transform has been developed where a one- or two-dimensional grating is used, which eliminates the need for scanning essential for the method that uses a Talbot interferometer.
The radiography using such a phase contrast method can image breast tissues, joint cartilages, and soft tissues around joints, which cannot readily produce clear images due to a low difference in absorption of X-rays in the absorption-contrast method. Thus, it is expected that medical costs and burdens on patients be reduced by using X-ray images produced by the phase contrast method using common radiography for diagnosis of lesions such as rheumatism in cartilages or soft tissues.
In some cases, images for diagnosis of rheumatism are taken for potential patients with rheumatism in order to prevent the occurrence of and to achieve early detection of this disease. In other cases, such images are taken for patients with rheumatism to check the development stage of this disease (e.g., effect of doses).
Nevertheless, unlike able-bodied people, a patient who already has a lesion in a joint such as rheumatoid arthritis has great difficulty in stretching the joint along a subject table by his/her own.
In radiography of a joint such as a metacarpophalangeal (MCP) or a proximal interphalangeal (PIP) joint, a radiographer needs to perform the positioning of the patient such that the irradiation axis virtually coincides with the top of the joint, which is a region of interest (ROI) to be radiographed. The patient, however, has difficulty in maintaining the posture by his/her own, and thus the region of interest might move after the radiographer leaves the patient for the operation of an emission switch. In addition, the subject might move during multiple scans and exposures required by the method that uses a Talbot (Talbot-Lau) interferometer.
The method using the Fourier transform, which needs only one exposure without scanning, also takes a long time to reach a predetermined dose of irradiation (i.e., a radiographing time), and thus movement of the subject within this time period leads to a blurred image.
Furthermore, the optimum position of patient fingers on the subject table (in particular, the position in the z direction) needs to be found for each patient by varying a bending angle of a joint, in order to obtain a highly visible image of a cartilage for the purpose of early detection of the disease. The present inventors, however, have found that a clear image of a joint, especially a cartilage, can be obtained irrespective of the above-mentioned variations depending on patients if a stretched joint of an examinee is fixed to the subject table (i.e., with a fingertip pulled), as a result of intensive studies.